Carbon fiber is used in various composite materials because of its excellent properties such as high strength, high elastic modulus and high electric conductivity. With the progress of electronic technologies in recent years, carbon fiber is expected to be used as an electrically conductive resin filler for electromagnetic wave-shielding materials or antistatic materials or as a filler in a resin for use in an electrostatic coating by using not only the excellent mechanical properties of the carbon fiber, which have been heretofore utilized, but also the electrical conductivity of the carbon fiber or carbon material. Furthermore, the carbon material is expected to be used as a field emission material for a flat display and the like using of its properties such as chemical stability, thermal stability and fine structure.
Conventional carbon fiber is produced as a so-called organic carbon fiber which is obtained by heat-treating and carbonizing fiber such as PAN-(polyacrylonitrile), pitch- or cellulose-based fiber. In the case of using this carbon fiber as a filler for fiber reinforced composite materials, the carbon fiber is preferably reduced in its diameter or increased in its length, thereby enlarging the contact area with the matrix so as to elevate the reinforcement effect. Furthermore, for improving the adhesion to the matrix, the surface of the carbon fiber is preferably not smooth and is roughened to some extent by subjecting the surface of the carbon fiber to a surface treatment such as oxidation by exposure to air at a high temperature or coating or the like.
However, the organic fiber used as the starting material of this carbon fiber has a diameter of approximately from 5 to 10 μm and therefore, the produced carbon fiber cannot have a small diameter and is limited in the ratio of length to diameter (i.e., aspect ratio). Under these circumstances, there is a demand for the development of carbon fiber having a small diameter and a large aspect ratio.
When resin is used for an automobile body or when resin, rubber or the like is used for an electronic device, the resin, rubber or the like is required to have electrical conductivity comparable to metal. Accordingly, there is a demand that the carbon fiber used as a filler material also has higher electrical conductivity so that the requirements demanded in various electrically conductive coating materials, electrically conductive resin and the like, can be satisfied.
In order to have higher electrical conductivity, the carbon fiber must be graphitized and thereby improved in electrical conductivity. To improve electrical conductivity, the carbon fiber is usually graphitized at a high temperature. However, even by graphitization, the carbon fiber cannot have electrical conductivity comparable to metal. If the amount of carbon fiber blended is increased to compensate for this insufficient electrical conductivity, the obtained composite material disadvantageously decreases in workability and mechanical properties. Therefore, it is necessary to further improve the electrical conductivity of the carbon fiber itself or enhance the strength by reducing the diameter.
With respect to the use as a field emission material, studies have heretofore been made on the field emission by the Spint method. However, the production process by this method involves many steps and although the carbon fiber used for the electron emitting part is conventionally processed to have a needle-like tip using Mo or the like, the chemical stability and the thermal stability are not sufficiently high as an electron emitting material of a display.
In the late 1980's, studies have been made on vapor grown carbon fiber (hereinafter simply referred to as VGCF) of which the production process is utterly different from that of the organic fibers.
This VGCF is known to be obtained from the vapor-phase thermal decomposition of a gas such as hydrocarbon in the presence of an organic transition metallic catalyst, and a carbon fiber having a diameter of from hundreds of nm to 1 μm is obtained.
For example, a method where an organic compound such as benzene is used as a starting material and an organic transition metal compound as a catalyst, such as ferrocene, is introduced into a high-temperature reaction furnace together with a carrier gas to produce VGCF on a substrate (JP-A-60-27700 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)), a method of producing VGCF in the free state (JP-A-60-54998), and a method of growing VGCF on a reaction furnace wall (Japanese Patent No. 2778434) are known.
According to these production processes, carbon fiber suitable as a filler material by having a relatively small diameter, an excellent electrical conductivity and a large aspect ratio can be obtained and in practice, carbon fiber having a diameter of approximately from 100 to 200 nm and an aspect ratio of approximately from 10 to 500 is mass-produced and used as an electrically conductive filler material in fillers for resin or in additive materials for lead storage batteries.
The VGCF is characterized by its shape and crystal structure. This fiber has a structure such that carbon hexagonal network surface crystals are stacked like annular rings to form a cylindrical shape, and the center part thereof forms a very narrow hollow moiety.
However, on a mass-production scale, VGCF having a small diameter of less than 100 nm cannot be produced.
Iijima, S., 1991, Nature, 354, 56, have discovered a multi-layer carbon nano-tube obtained from soot after the evaporation of a carbon electrode by arc discharge in a helium gas and this carbon fiber has a diameter smaller than that of VGCF. This multi-layer carbon nano-tube is a fine carbon fiber having a diameter of 1 to 30 nm, where, similarly to VGCF, carbon hexagonal network crystals are stacked like annular rings centered in the fiber axis and closed to form a cylindrical shape and the center part thereof has a hollow moiety.
This method using arc discharge is, however, not suitable for mass-production and not implemented in practice.
The vapor-phase process has a possibility of producing a carbon fiber having a large aspect ratio and a high electrical conductivity and studies are being made to improve this process with an attempt to produce a carbon fiber having a smaller diameter. U.S. Pat. No. 4,663,230 and JP-B-3-64606 (the term “JP-B” as used herein means an “examined Japanese patent publication”) disclose a cylindrical carbon fibril comprising graphite and having a diameter of about 3.5 to about 70 nm and an aspect ratio of 100 or more. The structure thereof is such that continuous layers of regularly oriented carbon atoms are disposed concentrically about the axis of the cylinder to form multiple layers, the C-axis of each carbon atom layer is substantially orthogonal to the cylinder axis, a thermal carbon coating deposited by thermal decomposition is not contained in the entirety, and the surface is smooth.
JP-A-61-70014 discloses a vapor grown carbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, where the thickness of the pyrolytic carbon layer is 20% or less of the fiber diameter.
These carbon fibers all have a smooth surface, and therefore, are poor in adhesive property, wettability and affinity, and when used as a composite material, the surface of the carbon fiber must be treated, for example, by thorough oxidation. Furthermore, when used as a field emission material, the tip of the carbon fiber must be thinned.